Physical Danger
Pulled discs, arthritis, backaches, and an endless variety of cuts and bruises are all par for the course for janitors everywhere. There are also about a thousand ways to slip and fall. Add in the fact that being on your feet all day and crouching over sinks, tables, and messes can have some long-term consequences, and you can see why it might be a good idea to do a little yoga or something.
There's also the little fact that you work around dirty, germy things a lot. Yes, you'll get gloves—but by the time you retire, you'll probably have experienced every germ-related illness known to man.